This disclosure relates generally to the field of directional drilling using steerable drilling motors. More specifically, the disclosure relates to methods and apparatus for automatically operating a drilling unit to cause a wellbore being drilled with a drill string using a steerable drilling motor to follow a selected trajectory.
Steerable drilling motors are used in directional drilling operations to cause a wellbore drilled through subsurface formations to follow a selected trajectory. To cause the trajectory to remain on a particular direction, the drill string may be rotated from the surface, causing the steerable motor housing to rotate therewith. Such rotation causes the drill string to drill the wellbore along a substantially continuous direction. To change the direction of the wellbore trajectory, the rotation of the drill string at the surface is stopped, and drilling progresses using only the rotation of a drill bit at the lower end of the drill string provided by the steerable motor. The motor may be operated, for example, by flow of drilling fluid therethrough. The drilling motor may have a bend in its housing, such that when rotation of the drill string is stopped, the wellbore trajectory turns in the direction of the inside of the bend in the motor housing. Such procedure is known as “slide” drilling, and may continue until wellbore survey information, such as may be obtained by a measurement while drilling (MWD) instrument disposed in the drill string, indicates that the wellbore trajectory has been reoriented to a new selected direction. At such time, rotation of the drill string may resume (so-called “rotary drilling”).
Various techniques are known in the art for improving performance of directional drilling operations using steerable drilling motors. See, for example, U.S. Pat. Nos. 6,802,378, 6,918,453, 7,096,979 and 7,810,584 all of which are issued to Haci et al. The techniques described in the foregoing patents include devices and methods for “rocking” the drill string during slide drilling and methods for changing from slide drilling to rotary drilling and back again, among other things.
What is needed is a method and system for automating the transition from rotary to slide drilling, maintaining a selected direction of the steerable drilling motor during slide drilling and operating the drill string to reduce incidence of “stalling” of the drilling motor by application of excessive axial loading thereon.